gtk_rc_get_style_by_paths() returns NULL if no matching style was
found and the default style should be used. Actually use the default
style in this case.
https://bugzilla.gnome.org/show_bug.cgi?id=630003
When dragging a window over a screen edge and dropping it there,
maximize it vertically and scale it horizontally to cover the
corresponding half of the current monitor.
Whenever a "hot area" which triggers this behavior is entered, an
indication of window's target size is displayed after a short delay
to avoid distraction when moving a window between monitors.
https://bugzilla.gnome.org/show_bug.cgi?id=606260
